A Richmond, Fredericksburg and Potomac freight train derailed in Prince William County early yesterday, delaying Amtrak passenger trains for up to six hours on the busiest rail route on the East Coast, authorities said.

The derailment occurred at 5:58 a.m. 25 miles south of Washington in the Featherstone section of Woodbridge, adjacent to a housing tract and just feet from a pipeline containing flammable liquids. No injuries were reported. About 300 yards of northbound and southbound track were destroyed.

The accident, which involved 11 cars of a 96-car freight train en route from Alexandria's Potomac Yard to RF&P's Acca Yard in Richmond, was the fourth incident involving a train in the county this year, said Capt. C. Hadden Culp of the county fire department.

Mechanical failure was believed to be the cause of yesterday's derailment, which involved cars 72 through 82 on the 6,000-foot train, said RF&P spokesman William E. Griffin.

Passenger service on Amtrak trains that use the line were delayed from 30 minutes to six hours. Amtrak operates seven northbound and seven southbound routes along the stretch of rail daily.

The longest delay occurred when an Amtrak auto train en route to Lorton from Sanford, Fla., was rerouted, said Amtrak spokesman R. Clifford Black.

Passengers on a northbound train headed for Washington from Richmond were bused, delaying their journey 30 minutes. Nine of the company's daily routes were affected, Black said.

RF&P representatives were at the site conducting an investigation yesterday. The Federal Railroad Administration is also expected to investigate, Griffin said.

The cost of the derailment was set at $500,000. The accident is believed to have occurred after a coupler connecting two of the train's freight cars failed and fell onto the tracks, resulting in the derailment of several cars. "We don't believe the track was a problem or that there was any improper handling by the crew," Griffin said.

The county fire department's John E. Medici said several cars ahead of the derailed cars contained hazardous materials including chlorine, acetylene and flammable and corrosive liquids.

Three of the derailed cars contained paper products and another contained a nonhazardous asphalt additive, which spilled on the tracks.

Amtrak reported that the rail line was cleared about 7 last night.